Shock absorbers are used in conjunction with automotive suspension systems and other suspension systems to absorb unwanted vibrations which occur during movement of the suspension system. In order to absorb these unwanted vibrations, automotive shock absorbers are generally connected between the sprung (body) and the unsprung (suspension/chassis) masses of the automobile.
The most common type of shock absorbers for automobiles is the dashpot type in which a piston is located within a pressure tube and is connected to the sprung mass of the vehicle through a piston rod. The piston divides the pressure tube into an upper working chamber and a lower working chamber. Because the piston, through valving, has the ability to limit the flow of damping fluid between the upper and lower working chambers within the pressure tube when the shock absorber is compressed or extended, the shock absorber is able to produce a damping force which counteracts the vibrations which would otherwise be transmitted from the unsprung mass to the sprung mass. In a dual tube shock absorber, a fluid reservoir is defined between the pressure tube and a reserve tube which is positioned around the pressure tube. A base valve is located between the lower working chamber and the fluid reservoir to also produce a damping force which counteracts the vibration which would otherwise be transmitted from the unsprung portion to the sprung portion of the automobile during stroking of the shock absorber.
Because the amount of damping forces the shock absorber generates causes variation in driving characteristics, it is often desirable to have a shock absorber in which the amount of damping forces generated by the shock absorber is variable. Various designs in the prior art have provided this variable damping force feature.
Shock absorbers which are used to provide variable damping often have a base valve which controls the flow of damping fluid between the lower working chamber and the fluid reservoir. While such base valves generally serve their intended purpose of controlling the flow of damping fluid between the fluid reservoir and the lower working chamber, they often do not adjust the flow of damping fluid in response to the desired damping characteristics which the shock absorber is to provide. In other words, the prior art base valves would typically allow approximately the same amount of damping fluid to flow between the lower working chamber and the fluid reservoir regardless of whether it was desired for the shock absorber to provide firm damping or soft damping.